1. Field of the Invention
The invention in general relates to the fabrication of integrated circuits, and more particularly to the fabrication of capacitors utilizing barium strontium titanate as the capacitor dielectric.
2. Statement of the Problem
A crucial problem of ULSI (ultra-large-scale integration) DRAMs (dynamic random access memories) is how to provide sufficient storage capacitance in the small memory cell area available. In conventional DRAM's this problem is addressed by providing capacitors with highly complex structures to maximize the area of the capacitor. Such complex capacitors require complex fabrication processes, leading to decreased yields and increased cost. An alternative proposed solution is to use dielectric materials of high dielectric constant. Metal oxide materials, such as barium strontium titanate, commonly referred to as BST, are known to be useful in making integrated circuit thin film capacitors having high dielectric constants. See for example, Kuniaki Koyama, et al., "A Stacked Capacitor With (Ba.sub.x Sr.sub.1-x)TiO.sub.3 For 256M DRAM" in IDEM (lnternational Electron Devices Meeting) Technical Digest, December 1991, pp.32.1.1-32.1.4, and U.S. Pat. No. 5,122,923 issued to Shogo Matsubara et al. However both these references use sputtering to deposit the BST, which is inherently hard to control. Such conventional processes, while useful in conventional silicon technology, when used in combination with capacitors that use metal oxides, such as BST, as the dielectric, result in capacitors that have relatively high leakage current, fatigue significantly, and generally have undesirable electrical properties. In particular, while the dielectric constant of bulk BST is of the order of 300 to 4000, the dielectric constant of thin films made according to the conventional processes is significantly lower. This is believed to be due to surface charges caused by defects and impurities in the films.
The process of spin coating has been used for making metal oxides such as barium titanate, strontium titanate, and barium strontium titanate. See G. M. Vest and S.Singaram, "Synthesis" of "Metallo-organic Compounds For MOD Powders and Films", Materials Research Society Symposium Proceedings, Vol. 60, 1986, pp. 35-42, Robert W. Vest and Jiejie Xu, "PbTiO.sub.3 Thin Films From Metalloorganic Precursors", IEEE Transactions On Ultrasonics, Ferroelectrics, and Frequency Control, Vol 35, No. 6, November 1988, pp. 711-717, and "Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method", Materials Research Society Bulletin, October 1989, pp. 48-53. In these papers, it was speculated that this process might have limited use for electronic devices, however, the grain size reported was 2000 .ANG. minimum, which is as large or larger than the film thickness typically used for capacitor dielectrics in integrated circuits, and the quality of the film was inferior to that produced by other processes, such as sputtering, and thus the spin-on process as applied to these metal-oxides up to now has been limited to inks for screen printing and other macroscopic purposes. A paper given by some of the present inventors disclosed using a spin-on process to deposit BST, but does not disdose any further details of the process, and the spin-on process used yields inferior results to other fabrication processes. See "Deposition of Ba.sub.1-x Sr.sub.x TiO.sub.3 and SrTiO.sub.3 Via Liquid Source CVD (LSCVD) For ULSI DRAMs", given at the 1992, International Symposium on Integrated Ferroelectrics, Mar. 9-11, 1992.
3. Solution to the Problem
The present invention solves the above problems by providing BST thin films in which the grain size is much smaller than the thickness of the film. The average grain size of the BST grains in the thin films according to the invention is about 40 nm. Typical film thicknesses of the dielectrics in integrated circuit capacitors are 100 nm to 200 nm. Thus the grain size in the BST thin films according to the invention is 2.5 to 5 times smaller than the film thickness. This small grain size results in capacitors with much improved electrical properties.
The process according to the invention preferably includes deposition of a liquid precursor by a spin-on process. Preferably the liquid precursor is an alkoxycarboxylate precursor as described in U.S. patent application Ser. No. 08/132,744 filed Oct. 6, 1993, which is hereby incorporated by reference. The liquid precursor is dried and annealed to form the BST. Preferably the annealing is performed at a temperature between 700.degree. C. and 850.degree. C.
In an exemplary embodiment, capacitors made with a dielectric material comprising BST thin films having the formula Ba.sub.1-x Sr.sub.x TiO.sub.3, where x is 0.03, were found to have a dielectric constant of nearly 500 and a leakage current of about 10.sup.-9 amps/cm.sup.2 when made by the process of the invention. The dielectric constant is about twice as large and the leakage current is about ten times as small as the best respective properties reported in the prior art for BST thin films. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.